Drawn and ironed metal cans such as aluminum two-piece cans or steel two-piece cans formed by integrally molding a can body and a can bottom section by drawing and ironing and then seaming a lid to the circumference of the open section of the can body have been widely used in the past. Metal can bodies for such two-piece cans have been manufactured by deep drawing a round sheet punched out from a flat sheet of aluminum or steel, forming a cup body in which a bottom section is integrated with a side wall, and then conducting ironing of the side wall of the cup body. By conducting ironing of the side wall, the thickness of the side wall of the cup body is reduced and a drawn and ironed metal can is formed by using the reduced quantity of metal materials.
The ironing is conducted by draw ironing in a wet state using a cooling and lubricant agent, that is, a coolant. The can molded by a wet molding process requires washing equipment and also environmental protection equipment, such as wastewater treatment equipment.
A synthetic resin coated aluminum can body or a synthetic resin coated steel can body (referred to hereinbelow simply as “resin coated can body”) in which a synthetic resin film such as a polyester film is laminated on both surfaces of a metal sheet has been suggested (See the patent document 1) because in such drawn and ironed metal cans, a coating on the inner surface for ensuring corrosion resistance of the inner surface of the can is not required and the can has excellent ability to preserve the flavor of its contents. Furthermore, deep drawing and ironing are conducted in a dry state without using a coolant, that is, a cooling and lubricant agent. This processing is called dry molding, and because a cooling and lubricant agent is not used, the manufacturing process is simplified and speed thereof is increased. Furthermore, because such a processing method reduces environmental load, it can be also referred to as an environmentally friendly method. Moreover, when printing is conducted on the front surface of the can body, the printing ink is not repelled by a lubricating film and adequate printing can be conducted. A resin coated can body is continuously manufactured at a high speed as a seamless can body, for example by a process in which a resin coated aluminum sheet in which a thermoplastic polyester resin is coated on both surfaces of an aluminum sheet is coated on the front surface thereof with a lubricant agent, then a cup body is formed by drawing in a dry state, and then one-stroke ironing of the cup body thus obtained is conducted in a dry state by using a punch together with a ring-shaped ironing die. It has also been suggested to circulate a heating liquid inside the punch and ring-shaped die before the continuous can manufacturing process is started and cause the flow of cooling liquid inside the punch and ring-shaped die immediately before and immediately after the continuous can manufacturing process is started, thereby maintaining the appropriate temperature of the punch surface and, at the same time, preventing the excess increase in can temperature at the initial stage of deep drawing and ironing, so that continuous ironing can be conducted.
Patent Document 1: Japanese Patent Application Laid-open No. 2002-178048, Paragraphs 0028-0035, FIGS. 3-6
FIG. 3 shows an example of the conventional process for manufacturing a metal can by deep drawing and ironing of a metal sheet such as aluminum sheet and an example of a general structure of the conventional manufacturing device. A deep drawing and ironing device 50 shown in FIG. 3 comprises a cylindrical punch 11, a cylindrical blank holder 12 into which the punch 11 can be inserted, an annular redrawing die 13 disposed in the vicinity of the end side of the blank holder 12 in the processing direction, a first ironing die 54, a second ironing die 55, and a third ironing die 56 disposed successively with a spacing therebetween at the distal end side of the redrawing die 13, and a stripper 17 disposed at the distal end side of the third ironing die 56. Those punch 11, blank holder 12, ironing dies 54 to 56, and stripper 17 are installed in a row on the same central axis line. A metal sheet is usually molded into a shallow cup C with a cupping press (cupper) that is not shown in the figure and supplied to the deep drawing and ironing device 50. When the punch 11 is inserted through the redrawing die 13, the cup C, which is sandwiched in an annular fashion and held by the blank holder 12 and the redrawing die 13 is molded by drawing into a redrawn can 60. Then, by inserting the punch 11, first to third ironings are carried out successively with the first ironing die 54 to third ironing die 56 on the side wall of the redrawn can 60 and the respective first-step can 61 to third-step can 63 are molded. The distance La between the redrawing die 13 and the first ironing die 54 is set with consideration for the length (length of the side wall) of the redrawn can 60, and the distance Lb between the first ironing die 54 and the second ironing die 55 and the distance Lc between the second ironing die 55 and the third ironing die 56 are set with consideration for the length of the first-step can 61 obtained in the first ironing process and the length of the second-step can 62 obtained in the second ironing process (both are the lengths of the side wall).
Because aluminum is a metal material with mechanical properties, such as strength, r value, and limit draw ratio, inferior to those of steel, a can body breakage, in which the can body is ruptured, easily occurs during drawing and ironing accompanied by large deformation. Therefore, a limitation is inevitably placed on the processing amount and processing speed in the process of drawing and ironing an aluminum sheet, and the speed in manufacturing an aluminum two-piece can and the gage-down of the material are also restricted.
As shown by an enlarged partial cross section of the cup C, when a drawn and ironed can is molded so that a sheet thickness reduction ratio from the original sheet thickness of the side wall is 60 to 80% by using a cup C manufactured from an aluminum sheet having no resin coating, the ironing ratio per one cycle of ironing in one ironing die has to be suppressed to 40% or less in order to suppress the breakage ratio of the side wall to 10 ppm or less. Therefore, as described hereinabove, the redrawn can (cup body) 60 has to be subjected to multistage ironing by successively passing through the ironing dies arranged successively in a row in the punch stroke direction. As a result, the punch stroke length tends to increase. For example, in the case of a 500 mL can, the three ironing dies have to be held at an arrangement length of 295.5 mm or more. As a result, the punch stroke length increases to about 668 mm. If the stroke increases, the inertial force and impact force generated in the movable sections of the can manufacturing machine increases, thereby facilitating the fracture of machinery parts. As a result, the can manufacturing speed is difficult to increase. Furthermore, because a long stroke causes increase in punch vibrations, the can manufacturing accuracy decreases, e.g., thickness deviation occurs, and a negative effect is produced on the can quality. Decreasing the stroke length and conducting simultaneous ironing in a plurality of ironing dies can be considered as measures for resolving this problem, but such measures are difficult to employ because rupture occurs at the side wall of the can.